We were working on the same thing in Hack a couple years ago & planned to add exhaustiveness. The __Sealed attribute enumerates allowed implementors so it's more obvious that a new implementation is a breaking change (i.e. syntactically changes the interface definition) docs.hhvm.com/hack/attribu...

I even used the term “oxidize” there: github.com/facebook/oca...

It felt more original in 2019 😅